The present invention relates to an asphalt drum mixer of the type employed to continuously heat and dry stone aggregate, while mixing the heated and dried aggregate with liquid asphalt to produce asphalt paving composition.
In one conventional drum mixer of the described type, the aggregate drying and mixing steps are carried out in a rotating drum which is inclined from the horizontal. The virgin aggregate is introduced into the upper end of the drum, and an outlet is provided adjacent the lower end of the drum for withdrawing the heated and dried aggregate. Thus as the drum rotates, the aggregate cascades through the interior of the drum and moves toward the outlet at the lower end. A burner is mounted adjacent one end of the drum so as to create a heated gas stream which moves through the drum, either parallel to or counter to the flow of the moving aggregate. Also, liquid asphalt is commonly introduced into the interior of the drum at a location midway along its length, so that the asphalt becomes mixed with the cascading aggregate and produces a paving composition which exits from the outlet.
With the increased need to control the emissions from such drum mixers, new designs have been developed wherein the virgin aggregate is heated and dried, usually in a drum of the counterflow type, and the heated aggregate is then mixed with liquid asphalt, and possibly also with recycled asphalt pavement (RAP) in a zone of the drum, or a separate device, where it is neither exposed to the hot wet gases nor the radiant energy of the flame used to dry and heat the virgin aggregate. A drum mixer of this type is disclosed in the U.S. Pat. Nos. 4,867,572 and 5,052,810 to Brock. More particularly, in the drum mixer of the Brock patents, a fixed sleeve surrounds the lower portion of the rotating drum so that the heated and dried aggregate is discharged into the annular chamber which is formed between the drum and sleeve. Also, an inlet is provided in the sleeve by which RAP may be introduced into the annular chamber, and another inlet is provided to introduce liquid asphalt into the annular chamber. The drum mounts mixing blades which move through the annular chamber to mix the materials and cause them to move longitudinally to the discharge outlet of the sleeve.
In drum mixers of the type disclosed in the referenced patents to Brock, the temperature of the virgin aggregate must be carefully controlled to ensure a predetermined final temperature of the asphalt composition after the addition of the liquid asphalt and RAP, if used, and under varying operating conditions. For this purpose, it has been conventional to position an infrared sensor or thermocouple through the shell of the mixer near the point where the virgin aggregate enters the annular chamber. The sensor signals the burner to control the flame intensity and thereby maintain the desired temperature of the aggregate.
In has been recognized that the temperature sensor must be shielded from the radiant heat of the burner in order to accurately measure the temperature of the aggregate. Also, the sensor must be exposed to a flow of representative aggregate, yet not be exposed to the full fury of the mixing zone where it can be quickly destroyed by abrasion and the forces imparted by the mixing blades. Heretofore, these requirements have not been fully met.
It is accordingly an object of the present invention to provide a drum mixer of the type which is useful in the continuous production of asphalt paving composition, and which has an inexpensive, reliable temperature sensing device which permits the temperature of the liquid asphalt being produced to be maintained within acceptable limits under varying operating conditions.
It is a further and more particular object of the present invention to provide a drum mixer of the type having an annular mixing chamber wherein stone aggregate is mixed with liquid asphalt, and which includes a reliable temperature measuring device which is protected from the radiant energy of the burner and from physical abuse in the annular mixing chamber, and which is subjected to a representative portion of the aggregate so as to be able to provide an accurate measurement of the temperature of the aggregate in the annular chamber.